350 A. W. GRABAU — PALEOZOIC CORAL REEFS 



the bedded Paleozoic limestones must be referred to this t3^pe of rock. 

 It is not always readil}" distinguishable from crinoidal limestone, which 

 must be considered an organic and not a clastic rock. In fact, these two 

 t3'pes of rock often grade one into the other. 



Among the varieties of calcarenite may be mentioned : 



1. Coral and shell sand rock — marine. 



2. Coral and shell sand rock — seolian (as in the case of the Bermudas). 



3. Limestone sand rock, practically indistinguishable from number 1. 



Calcarenites are not infrequently wholly barren of organic remains, as 

 is the case with ordinary sandstones. Some typical calcarenites of the 

 Traverse group of Alpena, Michigan, have analyzed over 99 per cent 

 CaCog. Others approach in composition pure dolomites. A perfect 

 gradation exists between this and the next group. 



CALCILUTITE 

 (Etymology : calx = lime + lutum = mud) 



A limestone or dolomite made up of rock flour, the composition of 

 which is t^^pically non-siliceous, though many calcilutites have an inter- 

 mixture of clayey material. They correspond to the pelites among the 

 siliceous rocks. The purest calcilutites, with a composition of 96 per 

 cent or over of CaCOg, or of CaCOg plus varying amounts of MgCog, have 

 generallj" a compact structure Avith a conchoidal fracture. The}^ ma}^ 

 be thin bedded or heavy bedded, in the former case not infrequently 

 showing ripple marks, as has been observed on an extensive scale in 

 Upper Siluric calcilutites on the Beaver islands, lake Michigan. The 

 rock here analyzed nearl}' pure calcium and magnesium carbonates.* 



Two chief varieties of calcilutite may be considered, namely : Rock 

 composed of the lime mud resulting from the trituration of organic de- 

 posits, such as coral reefs or shell or crinoid beds, and rock made of the 

 consolidated mud resulting from the erosion of older limestones. The 

 latter type appears to be exemplified b}^ the dolomitic calcilutites of the 

 Upper Monroe beds of Michigan, including those of the Beaver islands 

 already mentioned. As organic remains are rare in this entire forma- 

 tion, it must be assumed that the material of these limestones was de- 

 rived from the erosion of the underlying Niagara dolomites, unless, 

 indeed, we assume, as has been done, that the}^ are chemical sediments. 

 Calcilutites of a less pure character are found in the water limes of New 

 York, and probabl}' in those of New Jersey and Pennsylvania. Unless 

 these are considered of chemical origin, the source of the material com- 



* Reference to unpublished notes on the Michigan rocks is made by permission of the state 

 geologist, Dr A. C. Lane. 



