During the next era, the Proterozoic, the record of life becomes somewhat clearer. Fossils are hardly to be regarded as abundant but there were several well-defined types of animals which left shells and other parts composed of mineral matter. Among these may be mentioned the Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced delicate, often lace-like shells of many patterns adorned with the radiating filaments or spines which have suggested the name for this group. Foraminifera produced minute shells, sometimes many chambered, and often bearing a confusing resemblance to the work of snails. Common chalk is composed almost entirely of such shells and fragments of them. Sponges and Bryozoa are animals of slightly higher organization. They are many-celled instead of one-celled and the cells have special work to perform, which is a most important step in the direction of the specialization which characterizes the structural and life pattern of later arrivals. The Bryozoa lived in moss-like colonies which have been important rock-makers; the fossil forms bear some resemblance to corals. Sponges are too well known to require description although the familiar article of commerce is merely the framework of once-living animals. They represent the earliest organization of true animal bodies even though in appearance they may have a resemblance to plants. Actual plants of this era were of the algae class, aquatic in habit as were their animal neighbors, the first to leave a record in the form of fossils. This record, obscure and distorted, has long been a source of perplexity to investigators. Without well-defined floras and faunas to guide them, and with rocks frequently in chaotic relationships, early geologists were content to regard it all as a “Pre-Cambrian complex.” Recent studies have contributed a great deal of information not available some years ago. It is quite possible that more advanced types of life were in abundance before the close of the second era, but material on which to base sound opinion is still scarce. Rocks of Pre-Cambrian age are plentiful in the foothills region west of Denver. The schists, gneisses, and quartzites exposed for some miles immediately beyond the red-beds are part of this great complex. The Idaho Springs formation is known to be one of the oldest in this district, although its exact age has not been determined. Other formations are recognized among the metamorphic rocks of the region but none has contributed to our knowledge of early life. CAMBRIAN LIFE