THK PARALLEL ROADS OF OLEN ROT. 163 
two highest parallel roads stop abruptly at different points >^, 
near the mouth of Glen Roy, but no remnant of the barrier 
against which they abutted is to be seen. It might be urged 
that the subsequent invasion of the valley by glaciers has 
swept the detritus away; but there have been no glaciers in 
these valleys since the disappearance of the lakes Profes- 
sor Geikie has favored me with a drawing of Glen Spean 
"road" near the entrance to Glen Trieg. The road forms 
a shelf round a great mound of detritus which, had a 
glacier followed the formation of the shelf, must have been 
cleared away. Taking all the circumstances into account, 
you may, I think, with safety dismiss the detrital barrier as 
incompetent to account for the present condition of Glen 
Gluoy and Glen Roy. 
Hypotheses in science, though apparently transcending 
experience, are in reality experience modified by scientific 
thought and pushed into an ultra experiential region. At 
the time that he wrote, Sir Thomas Dick-Lauder could not 
possibly have discerned the cause subsequently assigned for 
the blockage of these glens. A knowledge of the action of 
ancient glaciers was the necessary antecedent to the new ^~V/ 
explanation, and experience of this nature was not pos- 
sessed by the distinguished writer just mentioned. The 
extension of Swiss glaciers far beyond their present limits, 
was first made known by a Swiss engineer named Venetz, 
who established, by the marks they had left behind them, 
their former existence in places which they had longforsaken. 
The subject of glacier extension was subsequently followed 
up with distinguished success by Oharpentier, Studer, and 
others. With characteristic vigor Agassiz grappled with it, 
extending his observations far beyond the domain of Switz- 
erland. He came to this country in 1840, and found in 
various places indubitable marks of ancient glacier action. 
England, Scotland, Wales, and Ireland he proved to have 
once given birth to glaciers. He visited Glen Roy, surveyed 
the surrounding neighborhood, and pronounced, as a conse- 
quence of his investigation, the barriers which stopped the 
glens and produced the parallel roads to have been barriers 
of ice. To Mr. Jarnieson, above all others, we are indebted 
for the thorough testing and confirmation of this theory. 
And let me here say that Agassiz is only too likely to be 
misrated and misjudged by those who, though accurate 
within a limited sphere, fail to grasp in their totality the. 
