Wootlworlh.J 120 [Ai.ril 19, 
The term C:iiiil)ri(l<;"e slales is used in this i>;i[)er to desigiiute 
the slates referred to by Professor Shaler, and also their extension 
in otiier parts of the Boston ]>asin where fossils similar to those 
found in tiie vicinity of the Mystic River occur. Tn g(Mieral it 
may be said that the series of slates described by Professor 
Crosby as l^'ing ui)on the Poxbury conglomerate fall into the 
Cambridge slate formation. 
Trails consisting of a narrow medial furrow and lateral ridges 
occui- on the upper surface of black shaly layers in Maiden and 
in the Mystic Piver and Clarendon Hill quarries in Somerville. 
Similar trails are made on the beaches of Boston outer harbor, at 
the present time, by the iso))od crustacean, Idotea irrorata. 
Pitted impressions, isolated, in groups, or in pairs, resembling the 
worm-borings known as Arenicolites, also occur widely dis- 
tribute<l in the forrnation, but cross-sections of the slate gen- 
erally fail to show the U-shaped tube on the existence of which 
the annelid origin of the impressions is determined. A cuj)- 
shaped depression resembling the form known as Monocraterion 
with a central vertical tube was found by the writer in the 
slate of Beacon Street in Newton Centre. A jointed trail from 
West Somerville, Mr. Walcott thinks, is probably that of an 
annelid. 
A single cast lying in the bedding plane of the slate in the 
Mystic Piver (piarries of Somerville suggests a species of 
Tlyolithes, but this and the forms above described are too 
indelinite to afford a determination of the age of the slates. 
A series of transversely striated trails or im])ressions of 
j)roblematical origin has been collected from the slates at Maiden 
and near Norfolk Down's station in WoUaston. Similar speci- 
mens were collected from the last named locality several years 
since by Professor Shaler. A discussion of these and other 
problematic fossils in tiie formation will be given in a forthcoming 
})aper, in the Bulletin of the museum of compnrative zoology. 
The fossils thus far found indicate that we have in the slate 
series one of those barren sections carrying annelid or crustacean 
trails, which occur in various horizons of the Paleozoic. The 
occurrence of shriidcage cracks and ripple-rnarks in the zones 
carrying these trails, together with the increasing amount of 
arenaceous sediment towards the west, indicates tolerably shallow 
water during the deposition of the slates. 
