38 THE REPORT OF THE No. 19 



Water containing free carbonic acid (derived from decaying vegetable matter) dissolves a 

 portion of carbonate of lime, but the bicarbonate thus formed is easily decomposed, even by 

 mere exposure to the atmosphere and a precipitation pf calcareous matter takes place. In this 

 manner the calcareous tufas of our swamps, (notably at Komoka), together with stalactites and 

 stalagmites are produced, and similar processes on a large scale have deposited the strata in 

 ancient seas and lakes. All limestones effervesce in acids ; but the dolomites effervesce only 

 feebly, unless the acid be heated. In some parts of Canada dolomites abound, but I think they 

 are best shown along the Grand River from Eloi'a to Gait, at Guelph on the Speed, and at 

 some places in Dumfries. It is of several shades, connnencing at the top with a yellow color, 

 and becoming lighter and semi-crystalline about 50 feet lower ; while about 100 feet from the 

 top of the series the rock is almost white, and granular in texture — a most beautiful stone. 

 Some thin beds are of a pale brown, still retaining the granular appearance. Between the 

 seuii -crystal line and granular textures there is, in some placei», a thin band of rock which will 

 powder in the fingers, and seems to be principally sulphate of magnesia. This, however, only 

 appears in places. The dolomites yield excellent building stones, especially in the semi- 

 ciystalline series, while the granular layers are chiefly burned for building lime. In fact, the 

 lime produced from these rocks is renowned for its excellence, becoming extremely hard with 

 age, and it is doubtful if as fine a quality can be produced from any other cla'ss of limestone. 

 The dolomites of the Guelph series are placed in the Onondaga Salt group of the Upper Silurian, 

 which derives its name from C)nondaga, near Syracuse, N. Y. In Canada the Onondaga deposits 

 attain between 200 and .■:{00 feet m thickness, and consist of thin-bedded dolomites, with greenish 

 shales (chieHy argillaceous dolomites), and some masses of gypsum. The latter does not occur 

 in regular beds, but in lenticular masses. The dolomites above the gypsum are generally arched 

 and more or less fissured, while those on which the gypsum rests retain their horizontal position. 



This Guelph formation is peculiar to Ontario, its rocks not having been traced beyond the 

 limits of the province. It follows the western limits of the Niagara, and occurs as a bean- 

 shaped mass, gradually thinning out westward toward Lake Huron, and eastward near 

 Ancastor. Its greatest thickness is about 160 feet. Its principal exposures are at the places I 

 have ab-eady mentioned. Along the Grand River it frequently presents vertical cliffs 80 feet 

 high. 



The fossils contained are small in variety, and most of them are identical with those belong- 

 ing to the Niagara beds, as Favosites gothlandica and Haly sites catenulatus, but others appear 

 to l)e confined to this formation. As a general rule the fossils are somewhat obscure, and for the 

 most part not very abundant. The most characteristic fossil, and one peculiar to this formation 

 is the Megalomus Canadensis, usually found in the form of internal casts, and I will confine 

 my attention to this fossil. 



The Megalomus belongs to the Lamellibranchiata, or conehifera order, which are marine or 

 fresh-water animals of the acephalus (headless) type. In the adult condition they have lamina- 

 ted gills or branchiie for breathing purposes, and they secrete a bivalve external shell. The two 

 valvesare nearly always of ecjual size, but always more or less inequilateral. (A line drawn straight 

 through the middle of an equilateral shell, divides it into two equal parts.) These mollusks are 

 exceedingly abundant in the fossil state, though not so numerous as the brachiopods in older 

 rocks. The existing species, both marine and fresh-water, number about 3,000, but about 

 double that number of fo.ssil forms have been found, which belong to many successive periods, 

 and it is very probable this is only a very small portion of the Lamellibranchiata fauna of the 

 past. 



In their classification these mollusks fall into two leading sections and four groups ; — 



1. Asiphonida. 



(a) Pleuroconcha. 



(b) f)rtlioconcha. 



