360 
throughout the vast series of beds which belong to those old de- 
posits ; whilst elsewhere the few forms of life hitherto discovered 
indicate what M. Barrande has called a “ Primordial Zone.” These 
successive formations have now been traced, and more or less exa- 
mined, in almost every region of the globe, and everywhere the same 
limited assemblage of organic remains has been established — the 
same total absence of any indication of terrestrial life — the same few 
generic types, chiefly of Crustacea, cephalopoda, brachiopoda, most of 
which have long since ceased to be, whilst one at least has survived 
every subsequent revolution, and is still living in the present day. 
On the other hand, it will no doubt be argued by those who take an 
opposite view, that the circumstances attending this reclassification of 
the older rocks of Scotland tend more than ever to teach the necessity 
of caution in the interpretation of negative evidence. The abundant 
existence, it will be said, of organic life during the ages of the Silurian 
deposit is beyond question. Yet all traces of it have been obliterated 
absolutely throughout a vast series of beds : in others, the indications 
are so exceedingly obscure that their character is altogether doubtful; 
whilst only in one or two thin seams of limestone, and in still rarer 
quartzite beds, has an unequivocal record been preserved of the highly 
organised and abundant molluscan life of the Silurian seas. 
Before passing from the Geology of Scotland, I must direct the 
attention of the Society to the very beautiful geological map of this 
city and its vicinity which has been lately published by the De- 
partment over which Sir B. Murchison presides as Director-General. 
The coal-basin, with its coal crops and faults, was the work of Mr 
Howel ; the rest was surveyed by Mr Geikie ; both these gentle- 
men being geologists of the Government Survey. The admirable 
care and exactness with which they have given the minutest details 
of a very varied and intricate district, is an excellent example of the 
high economical as well as scientific value we may anticipate of the 
geological survey of the country. 
The oldest formation in this sheet is the Lower Silurian , of which 
two small patches occur along the southern edge of the map. They 
belong to the great Silurian tract of southern Scotland, against 
which the upper Old Bed Sandstone and carboniferous rocks of the 
Lothians rest unconformably. There are at present known only 
two areas of Upper Silurian strata in Scotland, of which one occurs 
in the Pentland Hills, and is mapped in the present sheet. It con- 
