Page:Walcott Cambrian Geology and Paleontology II.djvu/231

 also informs me that Chapter XIII, which includes the Holothurioidea, was written by E. S. Goodrich, as stated in a footnote on page 218, Part III, of Lankester's Treatise on Zoölogy. This was also overlooked in citing from that portion of the treatise. Due credit will be given in the final paper on the subject of the Burgess shale fauna.

The crustaceans now found in the Burgess shale lived in quiet, relatively shallow waters swarming with life and readily accessible to the fauna of the open sea. In the preliminary study of the fauna I have distinguished 56 genera in collections from a block of shale not over 6 by 40 feet in area and 7 feet in thickness. Individuals of several species of crustaceans occur in large numbers at three horizons, notably Marrella splendens and Hymcnocaris perfecta. Trilobites, with the exception of the genera Agnostus and Microdiscus, are not abundant, although their tests almost make up calcareous shales a few feet below the base of the Burgess shale.

The compact, smooth, exceedingly fine-grained siliceous Burgess shale was deposited from relatively quiet, muddy water. At intervals this condition must have continued for some time as layers of the shale several inches in thickness have the crustaceans distributed irregularly through them. Where the shale is in thin layers with distinct lamination and bedding surfaces the fossils are more abundant but less perfectly preserved. The presence of carbonic acid gas at the surface of the mud has already been spoken of.

Owing to faulting and alteration of the shales by shearing the area available for collecting is limited to about 120 feet of outcrop on a steep slope of the mountain. This condition limits our information as to the original extent of this remarkable mud deposit. It was probably laid down in a small bay or lagoon in close connection with the shallow Middle Cambrian sea.

Mr. E. S. Larsen, Jr., of the United States Geological Survey, examined sections of the shale and from his notes the following is taken:

The microscopic examination of the thin section of the rock shows that it is very fine-textured—so fine that much of the material shows aggregate polarization. It is made up largely of white mica, which occurs in minute shreds or scales arranged parallel to the cleavage of the rock. Kaolinite is