A Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”, invited its members to submit photos each week of a stone, mineral or fossil in their collection that starts with a letter of the alphabet. The first Posts in this Series can be found here. The following are my Posts for “I” and “J”.
“I” is for “Ichnogenus Protovirgularia”. This argillite stone from Gemstone Beach contains a trace fossil shape that has been identified by a trace fossil researcher as belonging to the Ichnogenus Protovirgularia.
A trace fossil is an “ichnofossil” – they are identified by their shapes (not by what made the shapes as this may not always be known). The term is derived from the Greek word “ichnites” meaning “footprint”. An “ichnogenus” is a group of trace fossils with similar characteristics (a “genus” is an intermediate category between “species” and “family”). Ichnogenus Protovirgularia are trace fossil shapes consisting of “a small keel-like trail which is composed of an elevated median line and lateral wedge-shaped appendages alternating on both sides”, including lines of chevron shapes (as in this stone I found on Gemstone Beach). They were given this name in 1850 by Frederick McCoy, an influential Irish palaeontologist, who believed he was seeing the actual fossil of an ancestor of Virgularia mirabilis, the slender sea-pen, hence the term “proto-virgularia”. It was not until 1958 that the shape was identified instead as a trace fossil (and one which was made by quite a different animal).
It is thought that the trace is usually the product of locomotion (travel movement) produced by small bivalves, a burrow resulting from the rhythmic action of a muscular cleft-foot. On the southern coast, these traces are called worm-casts – it is thought that the casts were left as the worms tunnelled through mud and compacted the (excreted?) sediment behind them. It is not yet clear to me what left the trace on this Gemstone Beach stone, and I am not yet 100% convinced that this trace isn’t something other than Protovirgularia. More details on Ichnogenus Protovirgularia and trace fossil stones can be found here.
“J” is for “(Picture) Jasper” – Picture Jasper from Birdlings Flat.
Jasper is usually described as a form of cryptocrystalline silicon dioxide (quartz), as are a number of other rock types such as chalcedony, agate, carnelian, chrysoprase, chert and flint. “The Photographic Guide to Rocks & Minerals of NZ” defines “cryptocrystalline” as meaning crystals that are less than 0.001 mm in size, too tiny to see even using a hand lens. Jasper is distinguished by incorporating other materials that give it opacity (blocking the light) and colour. The Dorling Kindersley/Smithsonian book, “Rock and Gem”, states: “Brick-red to brownish-red jasper contains hematite; clay gives rise to a yellowish-white or grey, and goethite produces brown or yellow” (see scan of book page below). Names for types of jasper often refer to aspects of their structure or composition – banded, orbicular, moss, brecciated, or jasp-agate. But sometimes names relate to surface appearance and patterns – mottled, spider-web, egg pattern, and picture. Patti Polk, in her book “Collecting Rocks, Gems and Minerals”, defines picture jasper as containing “a dazzling array of colors and exquisitely detailed patterns that resemble skies, mountain vistas, desert landscapes, and forest horizons”. She also refers to “warm tones of tan, gold, yellow, blue, green, and browns all swirled together in strikingly outlined picturesque scenes”.
I found this small picture jasper on Birdlings Flat about three years ago. Its pastel tones, the contrasts between light and dark, and the way a number of small cracks and fault-lines break up the surface pattern all contribute to its desert-canyon-landscape-picture. [This stone was Stone Eight in the Lockdown Stones of the Day Series.]
For the next Post in this Alphabetical Series, see here. You can find the Index for the Series here.
TumbleStone 
8 thoughts on ““I” is for “Ichnogenus Protovirgularia” and “J” is for “(Picture) Jasper””